Wide band adjustable capacitive attenuators



March 1965 N. MARCHAND ETAL 3, 7

WIDE BAND ADJUSTABLE CAPACITIVE ATTENUATORS Filed May 28, 1962 INVENTORS NATHAN Mmzc HAND 6b Kwau PLESSET ML do;

ATTORNEYS United States Patent 3,176,250 WiDE BANE) ADJUSTABLE CAPACKTKVE ATTENUATORS Nathan Marchand, Greenwich, Conn, and Kiven Plesset,

New York, N.Y., assignors, by mesne assignments, to

The Singer Company, a corporation of New Jersey Filed May 28, 1962, Ser. No. 198,181 5 Claims. (Cl. 333-34) The present invention relates generally to wide band capacitive attenuators, and more particularly to capacitive attenuators applied to co-axial transmission lines.

Capacitive attenuators have been described in the proceedings of the Institute of Radio Engineers, June 1935, pages 578-593. In such attenuators, output is controlled by varying the spacing between two condenser plates, and hence the mutual capacity existing between the plates. When the separation between plates is large in such an arrangement, the rate of attenuation is 20.9 db per radius, assuming equal circular plates, i.e. 20.9 Z/a, where Z and a are separation and radius of the plates. A linear relation is maintained, as spacing varies, for a very wide range of spacings, and down to the point for which the plates are practically, but not quite, in contact, and where attenuation is substantially zero.

Where capacitive attenuators are required for application to co-axial transmission lines, the problem is introduced that the plates of the capacitor constitute mismatched terminations for the lines. According to the invention, the opposed ends of the transmission lines have been flared uniformly in respect to both inner and outer conductors, but at a slightly greater angle of flare for the outer conductor. By proper choice of flare angles the characteristic impedances of the lines may be maintained throughout the flares. However, the capacitive plates of the attenuators, i.e. the areas terminating the flares, constitute sharp discontinuities, and introduce reflections and poor standing wave ratios (SWR). To avoid this difliculty we terminate each line, at the capacitive plate, i.e.

' at the discontinuity, in a resistance equal to the characteristic impedance of the line. To simulate a uniform resistance circumferentially of the line, we employ plural spaced resistances, having values selected to equal, in total, the desired characteristic impedance. By means of the specified constructions the introduction of a capacitive attenuator into a co-axial line introduces no effective discontinuities, and except in respect to attenuation, does not appreciably or substantially modify the line characteristics, over a very wide frequency band.

It is, accordingly, an object of the invention to provide a novel capacitive attenuator for a co-axial transmission line.

it is a more specific object of the invention to provide a novel capacitive attenuator for a co-axial transmission line, which does not introduce substantial reflections or disturb standing wave patterns.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description when taken in conjunction with the accompanying drawings, wherein:

The single figure of the drawing is a longitudinal section of a capacitive attenuator in accordance with the invention, incorporated into a co-axial transmission line.

Referring now to the drawings, the reference numeral denotes one section of co-axial transmission line, and the reference numeral 11 another, arranged physically as an axial continuation of the one section. The line 10 includes an outer conductor 12 and an inner conductor 13, and the line 11 includes an outer conductor 14 and an inner conductor 15.

The inner conductor 13 includes a concentric flare 16 'ice which extends to a relatively large transverse area 17, having thereon a Teflon coating 18, which constitutes a condenser plate. The outer conductor 12 includes a flare 20 concentric with the flare 16, but having a greater flare angle, so as to maintain a characteristic impedance throughout the flare equal to that of the line 10. Resistances 21 are connected as terminations for the flares 16, 20. These are multiple and equally spaced about the circumference of the flare. Their sum equals the characteristic impedance Z of the line. For example, in the case of a 509 line three 1509 resistances may be distributed at 120 circumferential angles, or four 2009 resistances at circumferential angles.

The conductor 15 terminates in a flare 16a, identical with flare 16, and extending to a flat surface 17a which extends parallel with surface 17 and is of equal area and shape. The outer conductor 14, similarly, terminates in a flare 20a identical with flare 20, and terminating matching resistances 21a are connected between the conductor at the extremities of the flares selected to eliminate mismatch.

A cylindrical metallic shield 30 extends backwardly and forwardly of flare 20 and has the same diameter as the maximum flare diameter. The shield 30 extends at east to the small end of the flare 16, backwardly, and about four times that distance forwardly, i.e. over flare 20a. A further cylindrical metallic shield 31 extends from the large end of flare 20a to cover a considerable length of conductor 14. The metallic cylinders 30, 31 are preferably mutually contacting, but slidable with the transmission lines, to enable adjustment of attenuation, and specifically may be grounded, as are the outer conductors 12, 20 and 14, 20a. The conductors 30, 31 are effectively a shield which assures that the space between faces 17, 17a will be free of extraneous voltages. All the coupling between transmission lines 10, 11 takes place capacitively at the interfaces, 17, 17a, and no leakage or reflections occur at the coupling area, or adjacent thereto.

it has been found feasible to construct attenuators according to the invention which are operative over the frequency range 105,000 me. For maximum desired attenuation in one application a spacing of 2" was employed between faces 17, 17a, and provided an attenuation of 60 db. Minimum attenuation is approximately zero, for a spacing of .002", the thickness of coating 18.

Although we have illustrated the faces 17, 17a as flat parallel planes, the system does not require such construction. For example, Terman, Radio Engineers Handbook (McGraw-Hill Book Co. 1943) recommends a convex surface for one electrode.

While we have described and illustrated one specific embodiment of our invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What we claim is:

l. A capacitive attenuator for coupling co-axial transmission lines having co-axial transmission line inner and outer conductors and predetermined characteristic impedances, comprising flared extensions of said inner conductors, said extensions providing substantially parallel, terminating spaced interfaced capacitive plates of greater areas than the cross sectional areas of said inner conductors, flared extensions of said outer conductors arranged and adapted to maintain constant said characteristic impedance throughout said flared extensions, each of said outer conductor extensions extending to substantially the same plane as the terminating plane of its respective inner conductor extension, resistance terminations connected between the plates of said flared extensions'of said inner and outer conductors equal to said characteristic impedvary the spacing therebetween,

2. The combination according to claim 1 wherein at least one of said interfaced capacitive plates is coated with an insulating coating. 4 4

3; The combination according to claim 2 Whereinthere' is provided shielding for said plates. 4 I

4. The combination in accordance with claim 3 wherein said shielding comprisesa pair of concentric cylinders rel atively movable axially of the cylinders and each elec-' trically contacting one of said outer conductors.

5. A capacitive attenuator for'coupling co-axial trans mission lines having co-axial transmission line inner and outer conductors and predetermined characteristic impedances, comprising flared extensions of said inner conductors, said extensions providing substantially parallel,

I said outer 'conductor extensions extending to substantially the same plane as the terminating planeof its respective inner conductor extension, resistance terminations conqnected between the plates ofsaid flared extensions of said inner and outer conductors equal to said characteristic impedances, the space between said plates containing only dielectric, said plates being relatively movable at will to "vary the spacing therebetween, wherein said flared extensions of said inner and outer conductors have tapering spacings therebetween extending from said plates'to said inner and outer conductors, the taper being selected to V maintain said characteristic impedance throughout said planar terminating spaced interfaced capacitive plates of greater areas than the cross sectional areas of said inner conductors, flared extensionsof said outer conductors arranged and adapted to maintain constant said characteris- 1 tic impedance throughout said flared extensions, each of extensions. V V v 7 References Cited the file, of this patent UNITED STATES PATENTS Fernsler June 4, 1946 Hansen July 11, 1950 OTHER REFERENCES Harvey: Microwave Engineering, page 158, Academic Press, London and New York, (Copyright 19 63.) 

1. A CAPACTIVE ATTENUATOR FOR COUPLING CO-AXIAL TRANSMISSION LINES HAVING CO-AXIAL TRANSMISSION LINE INNER AND OUTER CONDUCTORS AND PREDETERMINED CHARACTERISTIC IMPEDANCES, COMPRISING FLARED EXTENSIONS OF SAID INNER CONDUCTORS, SAID EXTENSIONS PROVIDING SUBSTANTIALLY PARALLEL, TERMINATING SPACED INTERFACED CAPACTIVE PLATES OF GREATER AREAS THAN THE CROSS SECTIONAL AREAS OF SAID INNER CONDUCTORS, FLARED EXTENSIONS OF SAID OUTER CONDUCTORS ARRANGED AND ADAPTED TO MAINTAIN CONSTANT SAID CHARACTERISTIC IMPEDANCE THROUGHOUT SAID FLARED EXTENSIONS, EACH OF SAID OUTER CONDUCTOR EXTENSIONS EXTENDING TO SUBSTANTIALLY THE SAME PLANE AS THE TERMINATING PLANE OF ITS RESPECTIVE INNER CONDUCTOR EXTENSION, RESISTANCE TERMINATORS CONNECTED BETWEEN THE PLATES OF SAID FLARED EXTENSIONS OF SAID INNER AND OUTER CONDUCTORS EQUAL TO SAID CHARACTERISTIC IMPEDANCES, THE SPACE BETWEEN SAID PLATES CONTAINING ONLY DIELECTRIC, SAID PLATES BEING RELATIVELY MOVABLE AT WILL TO VARY THE SPACING THEREBETWEEN. 